1. Field of the Invention
This invention relates to a ball spline in which an outer cylinder is moved slidingly on and relatively to a ball spline shaft via balls.
2. Description of the Prior Art
A ball spline in which an outer cylinder member having a plurality of balls is moved linearly along a ball spline shaft has heretofore been applied to an industrial robot and a transfer machine. Such known ball splines include a ball spline provided with retainers for holding a plurality of balls, and a ball spline not provided therewith.
Known ball splines not provided with retainers include a ball spline shown in FIG. 16. This ball spline has a ball spline shaft 1 provided with a plurality of raceway grooves 2 in predetermined portions of an outer circumferential surface thereof so that the raceway grooves extend in the axial direction, and an outer cylinder body 3 provided with a hollow at which the outer cylinder body 3 is fitted slidably around the ball spline shaft 1. The hollow of the outer cylinder body 3 is provided in its inner circumferential surface with raceway grooves 4 opposed to the raceway grooves 2 in the ball spline shaft 1, and return passages 5. The ball spline further has spacer rings in which direction conversion passages 6 are formed, end caps 8, end seals 9, and balls 10 fitted between the opposed raceway grooves. The spacer rings 7, end caps 8 and end seals 9 are secured to both sides of the outer cylinder body 3 with bolts 11. In order to retain the balls 10, the width in cross section of the openings at the edge portions of the raceway grooves 4 formed in the outer cylinder body 3 is set smaller than the diameter of the balls 10 (refer to, for example, Japanese Patent Publication No. 61046/1991).
Although the ball spline shown in FIG. 16 has ball retainers at side edge portions of the raceway grooves 4 in the outer cylinder 3, it is difficult to form ball retainer-carrying raceway grooves 4, and, especially, finishing such raceway grooves is difficult. It is also difficult to accurately set the shapes of the raceway grooves 4 of ball load regions formed in the outer cylinder body 3, and direction conversion passages 6 formed in the spacer rings 7 and side rings 8.
The conventional retainer-carrying ball splines include a ball bearing shown in FIG. 17. This ball bearing comprises a ball spline shaft 1 provided with a plurality of raceway grooves 2 in predetermined portions of an outer circumferential surface thereof so that the raceway grooves extend in the axial direction, an outer cylinder body 3 provided in its inner surface with raceway grooves 4 opposed to those 2 in the ball spline shaft 1, and in its inner surface with inclined surface portions 12 adapted to change the direction in which the balls 10 roll, retainers 13 holding the balls 10 on the inner surface of the outer cylinder body 3, and a plurality of balls 10. Each retainer 13 is formed to a thin-walled seamless cylinder. The portion of the retainer 13 which is opposed to the balls 10 in a no-load region of the return passage 5 is formed to a larger height, i.e., on the diametrically outer side, and the portion of the retainer 13 which is opposed to the balls 10 in a load region to a smaller height, i.e., on the diametrically inner side. The retainer 13 is further provided with an elongated bore 16 so that the balls 10 rolling in the load region project slightly therefrom. The retainer 13 is fastened to the outer cylinder body 3 by screwing covering nuts 17 into both side portions of the inner circumferential surface thereof (refer to, for example, Japanese Utility Model Laid-Open No. 52317/1983).
The ball spline shown in FIG. 17 is provided at both of the axial end sections of a ball load region of each raceway groove 4 in the outer cylinder 3 with an escape portion formed by an inclined recess so as to guide the balls rolling in the direction conversion passage 6 in the retainer 13. Consequently, the forming of the raceway grooves 4 in the outer cylinder 3 becomes difficult, and the manufacturing cost increases. In a conventional ball spline, it is necessary that the raceway grooves in the outer cylinder body 3 in which the retainers 13 are provided be subjected to hardening. It is also difficult to form portions, in which the retainers 13 are provided, in the outer cylinder body 3, so that the cost of manufacturing the outer cylinder body 3 becomes high.
Ball splines in which retainers are formed in a divided manner as mentioned above include an endlessly sliding ball spline bearing (refer to, for example, Japanese Patent Publication No. 22209/1978) shown in FIGS. 18 and 19. In this ball spline bearing, torque transmitting no-load ball guide recesses 19 of a larger depth and torque transmitting load ball guide recesses 20 the depth of which is slightly smaller than that of the torque transmitting no-load ball guide recesses 19 are formed alternately in an inner surface of an outer cylinder 3, from which these depths are measured, in such a manner that these guide recesses extend axially. Both side portions of each load ball guide recess 20 has a radius of curvature close to that of balls. Each retainer 13 guiding the torque transmitting no-load balls 10 and torque transmitting load balls 10 is formed by equally divided segments 21. Each segment 21 is provided with return passages 23, which are opposed to the no-load ball guide recesses 19, on both sides of axially extending parting walls 22 positioned in the central portions of the no-load ball guide recesses 19, elongated bores 18 opposed to the load ball guide recesses 20 and wide enough to prevent the load balls from falling therethrough, and direction conversion guide passages 17 enabling the balls to move therebetween. After the segments 21 loaded with a plurality of balls 10 in the return passages 23 and on the elongated bores 18 have been fitted in the outer cylinder 3, rings are fitted around both end portions of all of the segments 21 to finish assembling the ball spline bearing. In this ball spline bearing, the inserting of the balls 10 and the fitting of the segments 21 in the outer cylinder 3 are difficult.